TeX Live contains command line programs like TeX, XeTeX, LuaTeX, dvips, and many others. It also contains a vast number of style files, class files, fonts, and documentation.

The command line programs must be compiled for individual platforms, and because that is a large undertaking, it only happens once a year. TUG also uses the yearly update to make changes in the underlying infrastructure of the distribution. You will obtain the current update by installing MacTeX-2018.

The style files, class files, fonts, and documentation do not need to be compiled, so they can be distributed as soon as they become available. TeX Live administrators update that part of the distribution daily. To obtain these updates, use TeX Live Utility, installed by MacTeX in
/Applications/TeX. Some TUG members update their personal copies just once a year, because the distribution is carefully tested for yearly updates. Other members update every day.

Ghostscript is developed independently by a different group of developers and has its own release schedule. We include the latest available release in the yearly MacTeX updates. When new releases of Ghostscript appear, a Macintosh install package is often provided shortly afterward at http://pages.uoregon.edu/koch.

The individual GUI programs in /Applications/TeX have independent update schedules.
Each uses the "Sparkle" open source update mechanism. Therefore in the the main program
menu of each program there is an item called "Check for Updates." Select this item to see if an update is
available, and to update if desired.

Since MacTeX is only released once a year, users who wait several months before updating
may get outdated versions of style files, of Ghostscript, and of the GUI programs.
So after installing MacTeX, it is useful to use TeX Live Utility and the Sparkle mechanism to make sure everything else is up to date.